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  Lms-based adaptive pre-distortion for enhanced power amplifier efficiencyUSPTO Application #: 20060082413Title: Lms-based adaptive pre-distortion for enhanced power amplifier efficiency Abstract: Systems and methods are disclosed to adaptively pre-distort a signal prior to being used by a non-linear circuit, such as a higher power amplifier (at transmit) or a low noise amplifier (at receive). The signal is compared with a feedback signal from the non-linear circuit and a metric is calculated and minimized. The input signal is adaptively changed, such as by varying tap coefficients, until the metric is sufficiently minimized, resulting in a pre-distorted signal that is substantially linear upon passing through the non-linear circuit. (end of abstract) Agent: Macpherson Kwok Chen & Heid LLP - San Jose, CA, US Inventor: Abhijit G. Shanbhag USPTO Applicaton #: 20060082413 - Class: 330149000 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20060082413. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] This application is a Continuation Application of U.S. patent application Ser. No. 10/704,439, filed Nov. 7, 2003. The contents are hereby incorporated by reference in their entirety. TECHNICAL FIELD [0002] This invention relates generally to communication systems and, more particularly, to pre-distortion circuitry for use with power amplifiers. BACKGROUND [0003] Many types of communication systems use a power amplifier as part of the front end of a transmitter for transmitting RF signals. In some systems, a low noise amplifier (LNA) at the front end of a receiver may be used for receiving RF signals. However, a power amplifier has non-linear distorting characteristics that can cause distortion of the information signal being amplified. The non-linear distorting characteristics of the power amplifier can impact the instantaneous amplitude and phase of the signal significantly. Furthermore, non-linearity within the RF components can introduce distortion in the signal and reduced SNR or poorer performance at the receiver. In wireless systems, non-linearity in the power amplifier at transmission can introduce intermodulation distortion which can result in spectral emissions in the adjacent channels. Typically, tight specifications in terms of spectral masks are provided by Standards Committees and/or Government bodies, which put a maximum limit to such spurious, out-of-band emissions for compliant transceivers. Therefore, it is desirable to provide a linear signal out of the power amplifier. [0004] One type of method to linearize the power amplifier output signal is to "pre-correct" the signal being input to the amplifier, also known as pre-distortion. There are many known techniques are used to pre-correct an information signal in order to linearize the output of the amplifier. One of these techniques involves amplitude correction which produces a linear piece-wise pre-correction function which is correlated to the non-linear characteristics of the amplifier. The result is a piece-wise correction curve which approximates the ideal correction. The correction is then added to the information signal. [0005] Pre-distortion has been almost exclusively in the baseband domain. The typical approach has been to apply a pre-distortion function at digital baseband. A relatively recent but commonly used approach has been to store the pre-distortion function as a look-up table which stores the gain and phase values as a function of the input signal envelope. The input signal is compared with the feedback signal from the power amplifier and certain metrics, such as ratio of in-band to out-of-band emission power and/or their correlation are used to adaptively map the look-up table with respect to the envelope of the input signal. Thus, the data path is simply a complex multiplication at digital baseband; the adapt-path (or feedback path) consists of a look-up table indexed by the input signal envelope and the adaptation metric is the ratio of in-band to out-of-band emission or the correlation between the feedback and the input signals. These operations are carried out at digital baseband. [0006] However, increasing the linearity of the power amplifiers can reduce the power efficiency of the amplifiers, make them more voluminous, require more cooling equipment, and substantially increase the cost and form-factor of the transceivers. To improve the linearity of the transmission without resorting to a higher end and more expensive power amplifier, one method that has been carried out in the prior art is to perform pre-distortion on the transmitted signal before the signal is input into the power amplifier such that the pre-distortion equalizes in some sense the non-linear post-distortion of the amplifier. The pre-distortion may be carried out within an integrated circuit; however, a significant challenge has always been to be able to adaptively obtain the pre-distortion transfer function so as to be optimal in some metric. It is also desirable to have a tracking mechanism with the pre-distortion so that different temperature and aging effects are also compensated for. [0007] Accordingly, it would be desirable to have systems and methods for performing pre-distortion in power amplifiers that overcome the disadvantages of the prior art as discussed. SUMMARY [0008] According to one aspect of the present invention, adaptive pre-distortion is performed using a least-mean square (LMS) method to adaptively obtain an optimal pre-distortion transfer function in the mean square error sense. In one embodiment, a LMS-based adaptive pre-distortion circuit coupled between an input signal to be transmitted and a non-linear power amplifier compares the input signal to a feedback signal from the power amplifier. The circuit adaptively reduces the least-mean square error between the two signals using both linear and non-linear filters and various feedback signals within the circuit. The result is an approximately linear output from the power amplifier. This technique can be used with single and multi-carrier transceivers, as well as with either a power amplifier (at the transmitter) or low noise amplifier (LNA) (at the receiver). The adaptive pre-distortion can be performed in the baseband domain or purely in the RF domain, using analog continuous-time or discrete-time signal processing. [0009] According to one embodiment, an LMS-based adaptive pre-distortion (LAPD) circuit receives an input signal to be transmitted, such as from a baseband processor and RF circuitry, and a feedback signal from a power amplifier. The LAPD circuit adaptively minimizes a metric, such as an error signal, formed by the difference between the two signals. The LAPD circuit receives the input signal through an adaptive AGC circuit. The LAPD circuit further includes a feedforward filter, an adaptive coefficient generator, a slicer, timing control circuit, and slicer output time-align circuit. An error timing align circuit within the adaptive coefficient generator may be used to time align the input signal from the adaptive AGC circuit with the error signal e(t) or equalize delays between the input signal and the error signal. The error timing align circuit may receive as an input signal a feedback tap coefficient vector c or the error signal e(t). The timing control circuit time aligns or equalizes the delays between the input signal and the feedback signal from the power amplifier to compute an error signal, which is the time difference between the two signals. This error signal is iteratively reduced until a desired signal is obtained. The output signal is then input to the non-linear power amplifier, which amplifies the signal for transmission, resulting in a linearized output signal. [0010] The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. BRIEF DESCRIPTION OF THE DRAWINGS [0011] FIG. 1 shows a block diagram of a portion of a single-carrier transceiver with adaptive pre-distortion according to one aspect of the present invention; [0012] FIG. 2 shows a block diagram of a pre-distortion circuit for use in the transceiver of FIG. 2 according to one embodiment; [0013] FIG. 3A shows a more detailed block diagram of the pre-distortion circuit of FIG. 2 according to one embodiment; [0014] FIG. 3B shows one embodiment of the pre-distortion circuit of FIG. 3A with a more detailed feedforward filter and adaptive coefficient generator; [0015] FIG. 4 shows one embodiment of a feedforward filter in FIG. 3A; [0016] FIG. 5 shows one embodiment of an adaptive coefficient generator of FIG. 3A; [0017] FIG. 7 shows one embodiment of a slicer input time align circuit for use in the system of FIG. 3A; [0018] FIG. 7A shows one embodiment of an interpolating mixer circuit of FIG. 7; [0019] FIG. 7B shows one embodiment of a phase detector circuit of FIG. 7; Continue reading... Full patent description for Lms-based adaptive pre-distortion for enhanced power amplifier efficiency Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Lms-based adaptive pre-distortion for enhanced power amplifier efficiency patent application. Patent Applications in related categories: 20080197925 - Distortion compensation amplifying apparatus - In an amplifying apparatus performing distortion compensation by imparting pre-distortion, a table for distortion compensation is efficiently updated for improving the accuracy of distortion compensation even when the frequency of occurrences of levels near a peak of an input signal is low. Compensation values in a distortion compensation table 2 ... ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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